1. Field of the Invention
One embodiment of the present invention relates to a benzofuropyridine compound or a benzothienopyridine compound. One embodiment of the present invention relates to a light-emitting element in which a light-emitting layer capable of emitting light by application of an electric field is provided between a pair of electrodes, and also relates to a display device, an electronic device, a semiconductor device, and a lighting device each including the light-emitting element.
Note that one embodiment of the present invention is not limited to the above technical field. One embodiment of the present invention relates to an object, a method, and a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, and a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a lighting device, driving methods thereof, and manufacturing methods thereof. In particular, one embodiment of the present invention may further include an organometallic complex.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting material has been actively developed. In particular, a light-emitting element called an electroluminescence (EL) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered applicable to a light source such as a backlight of a liquid crystal display and an illumination device.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. Specifically, by applying a voltage with a light-emitting layer provided between electrodes, electrons and holes injected from the electrodes recombine to raise the light-emitting substance to an excited state, and light is emitted when the substance in the excited state returns to the ground state. There are two types of the excited states: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a carrier-injection type light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. In contrast, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. Here, in a compound emitting fluorescence (hereinafter referred to as a fluorescent compound), in general, phosphorescence cannot be observed at room temperature, and only fluorescence can be observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
In contrast, the use of a phosphorescent compound can increase the internal quantum efficiency to 100% in theory. In other words, emission efficiency can be 4 times as much as that of the fluorescent compound. For these reasons, in order to obtain a highly efficient light-emitting element, a light-emitting element using a phosphorescent compound has been developed actively recently. As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal have particularly attracted attention because of their high phosphorescence quantum yield. For example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Documents 1 and 2.
An advantage of the use of the highly efficient light-emitting element is that power consumption of an electronic device using the light-emitting element can be reduced, for example. Energy issues have been discussed recently, and power consumption is becoming a major factor which affects consumer buying patterns; thus, power consumption is a very important element.
However, an energy level at which phosphorescence is emitted, a triplet excitation level, is located lower than a singlet excitation level with which fluorescence is emitted in terms of energy. Therefore, for a phosphorescent light-emitting element to obtain light having the same wavelength as a fluorescent light-emitting element, the phosphorescent light-emitting element needs a host material and a carrier-transport material which have a wider energy gap. However, such materials have not been well developed as compared with other materials.
Moreover, even with such a material having a wide energy gap, inherent emission efficiency of a phosphorescent element cannot be always achieved, and driving voltage increases in some cases depending on a combination of materials used in the layers.
In view of the above, in recent years, carbazole compounds, for example, have attracted attention as compounds having a wide energy gap. Patent Document 3 discloses a light-emitting element in which a carbazole compound is used for a host material in a light-emitting layer and a hole-transport layer. Patent Document 4 discloses a material having two carbazole skeletons.